Method for removing iodine from nitric acid

ABSTRACT

A METHOD FOR REMOVING SMALL AMOUNTS OF RADIOACTIVE IODINE FROM NITRIC ACID SOLUTION BY ISOTOPICALLY DILUTING THE IODINE, SPARGING THE SOLUTION WITH OZONE, AND DISTILLING THE IODINE FROM SOLUTION.

United States Patent 3,803,295 METHOD FOR REMOVING IODINE FROM NITRIC ACID George I. Cathers and Calvin J. Shipman, Knoxville, Tenn., assignors to the United States of America as represented by the United States Atomic Energy Commission No Drawing. Continuation-impart of application Ser. No. 231,820, Mar. 6, 1972. This application Dec. 21, 1972, Ser. No. 317,455

Int. Cl. C01b 7/14, 21/44 US. Cl. 423-390 8 Claims ABSTRACT OF THE DISCLOSURE A method for removing small amounts of radioactive iodine from nitric acid solution by isotopically diluting the iodine, sparging the solution with ozone, and distilling the iodine from solution.

CROSS REFERENCE TO RELATED APPLICATIONS This is a continuation-in-part of application S.N. 231,- 820, filed Mar. 6, 1972.

BACKGROUND OF THE INVENTION The invention described herein was made in the course of, or under, a contract with the United States Atomic Energy Commission.

This invention relates to the art of removing radioactive iodine from nitric acid solutions.

In reprocessing fuels from a liquid metal fast breeder reactor (LMFBR) it is desirable to dissolve the entire fuel element in nitric acid and to then separate the desired radioactive elements one at a time from the solution. The principal purpose of the reprocessing is to recover uranium and plutonium for reuse in another fuel element. One of the radioactive elements in solution is iodine as 1 and I. The proportions of the activities of these two isotopes vary with time in accordance with their half lives. In order to protect the environment and operating personnel from possible exposure to radioactive iodine, it is desirable to separate and isolate the iodine isotopes from solution prior to further processing. Radioactive iodine is particularly dangerous because of its volatility, and its ability to be carried by vegetation and transmitted through milk for human ingestion.

One prior art method for removing small quantities of a radioactive isotope from solution involves the use of a carrier. A carrier is a material which the radioactive element will tend to follow. The best carrier for a radioactive element is a nonradioactive isotope of the same element.

In the case of radioactive iodine it was generally thought that the iodine could be removed from nitric acid solution by first using conventional distillation followed by isotopic dilution with I as a carrier and repeated further distillation and isotopic dilutions until an acceptably low radioactive iodine concentration was achieved. However, it was discovered that this technique was generally ineffective for removing radioactive iodine from nitric acid solutions at molarities of less than about 10- Apparently upon reaching a lowiodine concentration of about 10- M, the remaining radioactive iodine is in a nondistillable form, with the result that only the diluent isotope is distilled 01f while the radioactive isotope remains in solution.

Reprocessing safety requirements will probably necessitate the removal of radioactive iodine to less than about 10- molarity for fuels aged less than 30 days and to less than 10- molarity for fuels aged more than 30 days. In view of the above safety requirement, the process of 3,803,295 Patented Apr. 9, 1974 using a diluent carrier isotope as taught by the prior art was unacceptable for achieving satisfactory iodine removal.

SUMMARY OF THE INVENTION It is thus an object of this invention to provide a process whereby radioactive iodine may be effectively and efficiently removed from nitric acid solutions to concen trations which are acceptable for reprocessing requirements.

This object as well as other objects is accomplished according to this invention by separating radioactive iodine from nitric acid solutions using isotopic dilution, ozone sparging, and distillation while sparging with a gas which will reduce nitric acid to an oxide of nitrogen.

DETAILED DESCRIPTION According to this invention it has been found that sparging an isotopically diluted solution with an ozone mixture prior to distillation, produces a solution in which the radioactive iodine is effectively removed upon distillation. The reason for this phenomenon is not entirely understood; however, the following non-limiting theory is presented as a possible explanation. The small amount of radioactive iodine remaining in solution after a conventional distillation is probably in a highly oxidized state, i.e., either in the iodic or periodic state. When the nonradioactive iodine diluent is added to the solution, it is in a lower oxidation state than the radioactive iodine. The differences in the oxidation states prohibit isotopic exchange from occurring. When the solution is sparged with an ozone mixture, the diluent isotope is oxidized to a higher state similar to that of the radioactive isotope, thus facilitating isotopic exchange.

Some of the radioactive iodine is also present as an organic iodide, such as methyl iodide. The organic iodides are extremely stable and do not release iodine upon distillation. The ozone sparge apparently oxidizes the organic iodides to leave the iodine in the iodic or periodic state, which will then release iodine upon distillation in accordance with the method of this invention.

As a part of the process of this invention, the iodine nitric acid solution is sparged with a gas which will reduce nitric acid to release an oxide of nitrogen (NO during distillation. The preferred sparging gas is N 0 however other such gases include N0 S0 H 8, and organic vapors such as CH OH. The improved results which are achieved by using this sparge are consistent with the theory presented above. In order for the iodine to distill as 1 it is necessary for the oxidized iodine species to be reduced. The nitrogen oxide sparge promotes the reduction of the oxidized iodine species thus making it possible for I to be released from the solution during distillation. It is preferred to carry out the sparging and distillation step at a temperature below the normal boiling point of the solution. At the normal boiling point of 4 M HNO, (108 C.) the solubility of NO;' gases is very low. The solubility, however, increases rapidly as the temperature is reduced. The desired chemical reaction is very dependent upon the NO concentra tion; hence operating at about 105 C. is preferred. However, the vapor pressure of the solution is reduced as the temperature is lowered so a lower limit is about C.

More specifically, according to this invention radioactive iodine is removed from nitric acid solutions by first distilling about 10 to 20 and preferably about 15% of the volume of a 3 to 6 molar aqueous solution of nitric acid. Under actual plant conditions this original solution may have a radioactive iodine molarity of about 10-. After the first distillation, the solution is isotopically diluted with carrier iodine, preferably by adding potassium iodide which readily oxidizes to I For the sake of convenience, the carrier iodine is added in an amount such that the original iodine concentration is restored. The amount of iodine added, however, must be sufiicient to provide a distillable level of iodine, i.e., an iodine molarity of at least 2.5x10- In order to maintain the nitric acid concentration at about 4 M, the original solution volume is preferably restored by adding water to the solution.

-At this point, the process of this invention comprises contacting the isotopically diluted solution with ozonized oxygen to promote isotopic exchange. Preferably, the oxygen-ozone mixture comprises about 2 volume percent ozone; however about 1 to 3 volume percent ozone may be used. The sparge is conducted at a rate of about 200 ml./minute and preferably 2x10- moles per minute for 250 ml. of solution, i.e., 10- moles O /minuteml., and at a temperature of 70 to 80 C. for a time period of from 20 to 30 minutes.

Conventional distillation equipment such as that described in copending application Ser. No. 231,820 may be used in the distillation steps of this invention. As mentioned above, improved results are achieved by sparging the solution with a nitrogen oxide, preferably N 0 during distillation. The sparge is carried out at a rate of about 250 ml./ minute and preferably about 4X 10 moles of N 0 per minute per ml. of solution moles/ minute for 250 mls.). The distillation is carried out at a temperature of 100 C. to 105 C. and preferably about 105 C. The normal boiling point of the solution is about 108 C. but it is preferred to carry out the distillation below this temperature, as is pointed out above. The isotopic dilution, ozonation and gas-sparge distillation are preferably repeated several times to reduce the iodine molarity to a desired level.

In order to further illustrate the process of this invention the following specific example is provided.

EXAMPLE A test solution of 250 ml. of 4 M HNO;, with 2.5Xl0- M I (added as KI with 11 tracer giving a radioactive iodine molarity of about 10 was distilled at 100 C. while sparging with 300 ml./minute of N 0 The solution distillation rate was 0.8 [ml/minute. The H O/N O mole ratio in the vapor phase was 3.3, and the residual 1 in solution after a 10 percent cut Was determined to be 0.10 percent, giving a decontamination factor of about '10 Water was added to replace the amount removed by distillation and the iodine concentration was reestablished at 2.5 l0 M by the addition of additional KI. The solution was then sparged for minutes at 75 C. with ozonized oxygen (2 volume percent ozone). The rate of sparging was 7x10" moles O /minute.

A second distillation step was conducted at 100 C. using N 0 sparging at 300 mL/minute. After a second 10 percent cut, the residual 1 in solution was determined to be 0.0069 percent giving a decontamination factor of 1.4x 10 The solution was again diluted to 250 ml., the I concentration increased to 2.5 x l0- M using KI, and then sparged at 75 C. for 20 minutes with ozonized oxygen at 7 10- moles 0 minute.

A third distillation step was performed at 100 C. As before, sparging was accomplished during distillation using 4 N 0 at 300 mL/minute. After a 10 percent cut, the residual 1 in solution was 0.0021 percent of the original concentration, giving an overall decontamination factor of 4.8 x10 This leaves a total radioactive iodine molarity of only about 2 10" In the above example the process of this invention yields an overall decontamination factor of 4.8x 10 after three distillations. When using the process as taught by the prior art under similar conditions using only isotopic dilution and distillation without any gas sparging, a decontamination factor of about 100 is achieved after three distillations. If only the N 0 sparge is used, a decontamination factor of about 1200 is achieved.

It is thus seen that the process of this invention not only overcomes the problems presented by the prior art isotopic exchange technique but provides a superior method by which radioactive iodine may be removed from nitric acid solutions.

What is claimed is: t

1. A method for removing radioactive iodine from nitric acid solution comprising the steps of isotopically diluting said solution with nonradioactive iodine to provide an iodine molarity of at least 2.5 10- sparging said solution with ozone, and at a temperature below the normal boiling point of the solution distilling iodine from said solution While sparging said solution with a gas which will reduce nitric acid, until the radioactive iodine concentration of the solution is less than l0 molarity.

2. The method according to claim 1 wherein said gas is an oxide of nitrogen.

3. The method according to claim 2 wherein said oxide of nitrogen is N 0 4. The method according to claim 1 further comprising diluting the nitric acid to its original concentration and repeating the isotopically diluting step, sparging with ozone, distilling and sparging with a reducing gas until a radioactive iodine concentration of less than 10- molarity is achieved.

5. The method according to claim 1 wherein said step of sparging with ozone is carried out at a rate of about 10 mole of ozone per milliliter of solution at a temperature of to C. and a time period of from 20 to 30 minutes.

6. The method according to claim 3, wherein said step of sparging with N 0 is carried out at a rate of about 4 l0 mole of N 0 per minute per milliliter of solution.

7. The method according to claim 2 wherein said step of distilling is carried out at a temperature between about C. and C.

8. The method according to claim 1 wherein said solution is 3 to 6 molar in nitric acid.

References Cited UNITED STATES PATENTS 3,053,644 9/1973 Huising 423500 OSCAR R. VERTIZ, Primary Examiner H. S. MILLER, Assistant Examiner U.S. Cl. XJR- 

